Tobacco addiction has a significant negative impact on the health and economic status of the individual and society. Nicotine is considered to be the primary reinforcing component responsible for tobacco addiction in human smokers [2]. Nicotine's main site of action in the brain is the nicotinic acetylcholine receptor (nAChR), which is composed of discrete combinations of various subunits. The a5 subunit has garnered recent attention based on genetic linkage studies suggesting polymorphisms in the a5 nAChR subunit gene (CHRNA5) increase susceptibility to nicotine dependence in human smokers [11, 12]. Further, insertion of this subunit into certain nAChR subtypes has been shown to alter receptor desensitization and activation kinetics in Xenopus oocytes [5]. The objective of this proposal is to determine the role ofaS-containing nAChRs in nicotine reward, dependence and withdrawal in mice. The intravenous self-administration procedure is considered to be the most reliable measure of the reinforcing properties of drugs of abuse. In Specific Aim I, I will examine the role of the a5 subunit in nicotine reward by examining intravenous nicotine self-administration in wildtype (WT) mice and mice with a null mutation of the a5 nAChR subunit gene (a5-/-). The intracranial self-stimulation procedure (ICSS) is considered to be a direct measure of the functioning of the brain's natural reward pathways. Thus, in Specific Aim II, I will investigate the role of a5- containing nAChRs in regulating the stimulatory effects of acutely administered nicotine on brain reward systems by measuring nicotine-induced lowering of ICSS thresholds in WT and a5-/- mice. Finally, in Specific Aim III, I will examine the role of the aS-containing nAChRs in the expression of reward deficits during spontaneous nicotine withdrawal in WT and a5-/- mice;elevation of ICSS thresholds during spontaneous nicotine withdrawal will serve as a measure of the reward deficit associated with nicotine withdrawal. I hypothesize that aS-containing nAChRs play an important role in regulating the actions of nicotine on the brain reward circuits, and thereby regulate nicotine self-administration behavior, the acute reward-enhancing effects of nicotine, and the reward deficit associated with nicotine withdrawal. Together, these data may reveal fundamental insights into the neurobiological mechanisms underlying nicotine reward, dependence and withdrawal. Importantly, these findings could lead to the development of novel therapeutics efficacious for smoking cessation and/or relapse prevention in humans.